If you go to The Field Museum in Chicago, you can see Sue, the largest, most complete Tyrannosaurus rex skeleton in the world. Sue is an impressive specimen, standing more than 13 feet (3.9 meters) tall and 42 feet (12.8 meters) long. Many dinosaur fossils have missing or broken bones, but Sue is more than 90 percent complete. Sue is also a real skeleton, not a cast. With the exception of the skull, on display elsewhere in the museum, the bones you can see are the real 67-million-year-old deal.

Paleontologist Sue Hendrickson discovered the dinosaur's bones in South Dakota on Aug. 12, 1990. Since then, researchers have figured out a lot about Sue's life. In spite of breaking some ribs, Sue lived to a relatively old age for a dinosaur, at least according to signs of wear on the bones. Researchers also estimate that the dinosaur weighed 7 tons (6.4 metric tons), about as much as a male African elephant. But what scientists don't know is whether Sue was male or female.

With just about all animals that use sexual reproduction, it's pretty easy to figure out which sex is which. All you have to do is observe their behavior or take a look at their sex organs. Figuring out the sex of a dinosaur is a little tricky, though. There aren't any around today, so you can't observe their behavior. Since sex organs are made of soft tissue, they don't fossilize well. On top of that, most paleontologists believe that dinosaurs, like birds, used one opening called a cloaca for reproduction and expulsion of liquid and solid waste. So even if researchers find a dinosaur fossil with well-preserved skin, they still may not know its sex for sure.

­DNA -- a master reference for figuring out what's what -- isn't much help with dinosaurs. While you can usually tell male and female mammals apart by their X and Y chromosomes, reptiles don't follow those rules. In many reptilian species, the temperature of the egg, not the chromosomes inside it, determines sex. Plus, DNA breaks ­down during fossilization, leaving little for researchers to analyze.

But that doesn't mean figuring out the sex of a dinosaur fossil is impossible. Next, we'll take a look at some of the theories behind frills, crests and bones, and how they might shed some light on whether that T. rex is a boy or a girl.

Telling the Difference Between Dinosaur Sexes

Telling male and female dinosaurs apart is easier said than done. Every once in an extremely long while, researchers might find a skeleton with eggs inside it -- a pretty good clue that one was female. One such find cropped up in Jiangxi Province, China, in 2005. But eggs aren't always the best indicator. It might seem like a skeleton perched atop a nest of eggs belonged to a female, but it's just as likely that male dinosaurs, like male emus and ostriches, were the ones that did the brooding.

Another potential way of dividing males from females is through studying sexual dimorphism. Think of peacocks who display huge, ornate tails in front of nondescript peahens. It could be that differences in neck frills and head crests had to do with a dinosaur's need to attract a mate. The size and proportion of a dinosaur's body may have been sex-related, too. Paleobiologist Phil Senter hypothesizes that the ultralong necks of Diplodocus and Apatosaurus may have evolved through sexual selection, with longer-necked dinosaurs attracting more -- or better -- mates [source: Atkinson].

The only trouble with this theory is that, even if there are sex differences in dinosaurs, we still don't know which sex is which. Did a female Triceratops use its neck frill to attract a mate, or was it the other way around? Were horns and tail spikes for defense against predators or for combat with rival males? Without preserved samples of sex organs or a time-traveling look at dinosaur behavior, it's impossible to say.

Aside from the serendipitous find of a dinosaur fossil filled with eggs, researchers have found one way to figure out if a dinosaur is female, at least if she's pregnant. In 2005, Dr. Mary Schweitzer, Jennifer Wittmeyer and John R. Horner reported the discovery of medullary bone in a Tyrannosaurus rex fossil in the journal Science. In female birds, medullary bone lines the interior, or medullary cavity, of bones. Birds use this bone as a reservoir for the calcium they need to produce eggshells. In all likelihood, the T. rex whose bones the team was studying was both female and pregnant. Unfortunately, researchers can only determine the sex of dinosaurs this way by breaking into the interior of the bone. On top of that, it only works if the dinosaur is pregnant.

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Samples of medullary bone: from left to right, T. rex, emu and ostrich

With groundbreaking discoveries like Schweitzer's and the well-preserved fossils being unearthed in places like China, it may one day be possible to make a pretty good guess about a dinosaur's sex. But for right now, it's a shot in the dark. To read more about paleontology, dinosaurs and scientific unknowns, read through the links on the next page.